Although digital media such as audio CDs and CD-ROMs allow perfect reproduction of digital data, the problems of controlling unauthorised reproduction are to some extent mitigated by the fact that these media are generally available in read-only form, so that the potential infringer needs specialist recording and CD-pressing equipment to make high quality copies.
However, the introduction of Digital Versatile Disk (DVD) technology, which aims to make low cost digital recorders widely available, has raised the need for sophisticated copy protection systems, to prevent extensive piracy. With the increasing provision of information, entertainment and other contents in digital form, for example digital video broadcasting (DVB) by satellite, cable television and digital terrestrial television (DVB-T) as well as through networks such as the Internet, DVD recorders, known as DVD-RAM recorders, have been developed to record such contents in digital form. Eventually, it is envisaged that such recorders will replace the various different forms of currently available storage equipment, including computer hard-disk drives and video cassette recorders.
The principles of DVD are well established, with DVD-RAM recorders such as the Hitachi GF-1000 series available on the market. Reference is directed to the substantial information on the Internet relating to DVD principles and products, including at &lt;http://fp97.inet-images.com/dvd/index.html&gt; and &lt;http://www.videodiscovery.com/vdyweb/dvd/dvdfaq.html&gt;, and to "DVD Demystified", by Jim Taylor, published by McGraw-Hill.
Without any form of copy control, digital data received via television or over the Internet can be recorded by a DVD-RAM recorder or other digital recorder onto a digital recording medium such as a recordable DVD disk, from which it can be further copied numerous times onto other DVD disks, without any degradation in the copy quality.
A method that prevents unauthorised copying by embedding copy control signals in data prior to transmission, has been proposed by Nikkei Electronics, and is detailed on the Internet at http://www.dvcc.com/dhsg/CFP-v.10.rtf. This is a response to a Call for Proposals issued by the Data Hiding SubGroup of the Copy Protection Technical Working Group, Version 1.0, July 1997, at http://www.dvcc.com/dhsg.
The proposed method uses a copy control signal which may take one of four possible values: Copy-Free, indicating that the contents can be copied freely, Never-Copy indicating that the contents can never be copied, Copy-Once, indicating that the contents can be copied only once and No-More-Copy, indicating that the contents have already been copied once and that further copies cannot therefore be made.
The structure of the transmission apparatus 1 for the proposed method is shown in FIG. 1. A playback module 2 reads data from the medium 3 on which it is recorded and sends it to the copy control signal embedding module 4. The embedding module 4 obtains the copy control signal to be embedded from the copy control information database 5, embeds it into the data using a suitable technique, for example, by applying a digital watermark, and sends the data to the transmission module 6. The transmission module 6 then transmits the data in a conventional way, for example broadcasting it via digital satellite, digital terrestrial television or digital cable television.
The structure of the recording apparatus 7 for the proposed method is shown in FIG. 2. A data enabling gate 8 lies between an input signal processing module 9 and a recording module 10, the gate 8 only allowing data to pass to the recording module 10 in accordance with the copy control signal determined by the copy control signal detection module 11. If data is allowed to pass to the recording module 10, it is recorded to an appropriate recording medium 12, such as a recordable DVD disk.
FIG. 3 is a flowchart showing the detailed operation of the recording apparatus 7. The input signal processing module 9 reads data input to the recording apparatus 7 at step s1 and sends it to the copy control signal detection module 11, which detects the copy control signal embedded in the data at step s2. At step s3, the signal detection module 11 determines if the Copy-Free signal is present. If it is present, control proceeds to step s4 at which a signal is sent to the data enabling gate 8 which in turn allows the data to pass to the recording module 10. Control passes to step s5 at which the data is recorded to the recording medium 12. The sequence of recording apparatus operations stops at step s6.
If, on the other hand, the Copy-Free signal is not detected at step s3, step s7 is executed at which the signal detection module 11 determines whether the Copy-Once signal is present. If the Copy-Once signal is not detected, the only possibilities remaining are that the copy control signal is set to Never-Copy or No-More-Copy. In either case, recording is not permitted, so control passes to terminating step s6. In this case, no enabling signal is sent to the data enabling gate 8, so that the received data is unable to pass to the recording module 10.
In the event that the Copy-Once signal is detected at step s7, control passes to step s8, at which an enabling signal is sent to the data enabling gate 8 which in turn allows the data to be sent to the recording module 10. Control then passes to step s9, at which the recording module 10 changes the embedded Copy-Once signal to No-More-Copy, so that further recordings are prevented. Step s5 follows, at which the recording module 10 records the data, including the new embedded copy control signal, onto the appropriate recording medium 12.
As a result of the above operations, the data on the recorded medium 12 contains one of three possible copy control signals: Copy-Free, Never-Copy or No-More-Copy. The Copy-Once signal is eliminated, so that further recording is only permitted if the data was originally designated Copy-Free.
While this method of copy protection is suitable where all recording devices include copy control signal detection, a problem arises when data is recorded onto devices which do not have such detection facilities. For example, referring to FIG. 4, with the continual increase in the speed and storage capacity of personal computers, it is possible to use a conventional digital receiver 15 linked to a personal computer (PC) 16 to receive digital data marked as Copy-Once. The data can be stored on the computer's hard disk drive (HDD) 17 as received, namely with the Copy-Once signal still embedded. When the data stream from the hard disk 17 is subsequently sent through the PC 16 to the DVD-RAM recorder 18, the recorder acts according to the appropriate branch of the flow chart in FIG. 3 (s3-s7-s8-s9-s5), changing the Copy-Once signal to No-More-Copy and making a single recording of the data stream onto, for example, a recordable DVD-video disk 19, from which no further copies can be made. However, since the original data is stored in Copy-Once format on the hard disk 17, this recording process can be repeated many times to produce a large number of high quality copies on individual disks 19.